Reflective projection system



Feb. 2, 1943., E. G. RAMBERG REFLECTIVE PROJECTION SYSTEM Filed Nov. 29, 1940 2 Sheets-Sheet 1 Feb. 2, 1943. E. G. RAMBERG 253099783 REFLECTIVE PROJECTION SYSTEM Filed Nov. 29, 1940 Sheets-Sheet 2 Z- 4 N! a /16 |-.l 4 I M I 12 3nventor Edward flabeylq Patented Feb. 2, 1943 REFLECTIVE PROJECTION SYSTEM Edward G. Bamberg, Haddon Heights, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application November 29, 1940, Serial No. 367,817

4 Claims.

My invention relates to projection systems of the type comprising a spherical mirror and a correcting plate for correcting spherical aberration. It relates in particular to systems of the said type as applied to the projection of images produced by a cathode ray tube.

An object of the invention is to improve the quality of a projected image. More specifically, it is an object of the invention to improve the contrast of such an image.

It has been found that if the central portion of the spherical mirror in a system of the abovementioned type is made nonreflecting, the contrast of the image on the projection screen is greatly improved. The reason for this is that, if the central portion of the mirror surface is utilized, it reflects light onto the object surface and this light is reflected back to the spherical mirror and projected onto the image screen. Thus, a false brightness is introduced into the image and the contrast is reduced.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which:

Figure 1 is a view showing one embodiment of my invention as applied to a television projector,

Figure 2 is a. view which is referred to in ex-v plaining the invention,

Figure 3 is a view showing anotherv embodiment of the invention,

Figure 4 is a view which indicates the distriburather than a fluorescent screen.

tion of light reflected from the spherical mirror,

and

to a system including a difierent type of cathode ray-tube.

.Referring to Fig. l, there'is shown a television projection system comprising a cathode ray projection tube l0 and a spherical mirror II and correcting plate I2 for projecting the cathode ray tube image or object upon a projection screen (not shown). The cathode ray tube image appears on a screen I3 which, in the example illustrated, is a coating of fluorescent material on the inner surfaceof the end of the tube envelope. This end of the tube preferably has a spherical surface with a radius r.

The cathode ray tube I0 may be of any suitable type and, therefore, need not be described in detail. Also, the image being projected may be produced by means other than a cathod ray tube, the feature of masking off the central portion of the mirror not being limited to television. F r

Figure 5 is a view showing my invention applied I Theoptical system comprising the mirror II and the correcting plate I2 is 01' the same type as that described and claimed in Patent No. 2,273,801, issued February 17, 1942, in the name of Daniel 0. Landis, and entitled "Television Receiver. The spherical mirror II by itself would project an image having a large amount of spherical aberration. This aberration is removed by the correcting plate I 2 which is properly shaped or figured to give an image or high quality on the projection screen. The particular shape of the correcting plate illustrated is such that it moves the normal focus of the edge rays from the mirror I I away from the mirror while it moves the normal focus of the paraxial rays from the mirror I I towards the mirror, the focus for all rays being moved to the plane of the projection screen. As stated in the above-identified Landis patent, the use of the correcting plate with the spherical mirror is based upon the principle 01' the wellknown Schmidt camera. In accordance with the present invention, the central portion of the mirror II is made nonreflecting as by means of a nonreflecting area or mask I6. The central area of the mirror represented by the mask l6 may be made nonreflecting in any other suitable manner as by painting this area of the mirror a dead black or by leaving the area unsilvered. In some cases, it may be preferred to cut out the central area of the mirror. The reason for the nonreflecting area I6 will be evident from an inspection of Fig. 2. In this figure, the paths of certain light rays reflected from the central portion of the mirror have been traced. It will be noted that some of the light rays are reflected back to the object or fluorescent screen I3. As indicated at B and B, such rays are reflected from the object whereby they cause a false brightness and reduce the contrast of the image on the projection screen.

It can readily be shown by tracing light rays as has been done in Fig. 2 that all reflections from the mirror back to the object will be avoided if the nonreflecting area I 6 is made to coincide with the area obtained by projecting the object area on the mirror from the center of curvature C of the mirror I l. Specifically, the area of the mirror falling within the boundary represented by the dotted lines I! (Figs. 1 and 2) is the said projected area and should be nonreflecting if the best contrast is to be obtained. Assuming that the tube III has the usual envelope of circular cross section, the diameter of the area I is twice the diameter of the fluorescent screen I! or substantially twice the diameter oi. the end of the tube when this particular design is employed.

It will be apparent that there is a certain loss in useful light if.an area as large as that just described is masked oil. In systems where this light loss would result in a projected image too low in illumination, a compromise design may be the preferred one, the area of the nonreflecting area being reduced to a value such that an image of satisfactory brilliance appears on the projection screen,

However, reduction in the nonreflecting area beyond a certain amount will not improve the light output from the center of the image but will continue to impair the contrast. This is illustrated in Fig. 3 where parts corresponding to those in Fig. 1 are indicated by the same reference numerals.

In Fig. 3, the nonreflecting area 16a is the area of the tube I at its maximum diameter projected onto the mirror II as indicated by the parallel dotted lines 2|. In the example illustrated, the area lid is a circular area having the same diameter as the end of the tube It. Decreasing this area lBa would not increase the amount of projected light from the center of the image because all light reflected from this area is blocked by the cathode ray tube itself. It would, however, decrease the contrast for the reason previously explained.

From the foregoing, it will be understood that the nonreflecting area of the mirror should lie either at or between the limits illustrated in Figs. 1 and 3 for most effective use of the invention.

The way in which tube ll will block light reflected from an area lBa having the same diameter as. the maximum diameter of the tube, is illustrated in Fig. 4. Here, the reflected rays M and N from the edge and center of the object l3, respectively, are shown blocked by the tube. It will be apparent that all rays from the center of the object l3 are blocked by the tube III while part of the rays from the other parts of the object are blocked.

Fig. 4 also illustrates the fact that, in the projected image, the light for the central portion of the image is obtained largely from an inner annular area of the mirror, while the light for the outer or edge portion of the image is obtained largely from an outer annular area. This is apparent from the reflected paths of the center ray N and the edge ray M traced in Fig. 4. For instance, if the ray N struck a point on the mirror ll closer to the mirror edge, the reflected ray would not pass through the correcting Plate II.

It follows that the optical system and the object must be properly dimensioned if the projected image is to have uniform illumination, i. e., if it is not to be brighter at the edges than at the center, or vice versa. Among other things, the diameter of the correcting plate l2 with respect to the diameter of the mirror II should be increased if the area of the nonreflecting portion of the mirror is increased.

By way of example, approximately the following relative dimensions were-employed in a system where the-mirror diameter D was 30inches, where R is radius of curvature of the mirror, d is diameter of spherical object surface, r is radius of the object surface, :1: is diameter of the correcting plate, and f is focal length of the central portion of correcting plate:

of the metal section Slb and has the same curvature as the end of the tube shown ing figures. V

The optical system is the same as previously described, but in this arrangement the spherical mirror H has an opening in the center through which the neck of the tube 35 extends.

A nonreflecting mask 36 is provided to improve the contrast just as in the other embodiments. In the particular example shown in Fig. 5 it happens that the projected area of the object 32 on the mirror it as projected from. the center of curvature C is the same as the projected area of the maximum diameter of the tube 3i projected by parallel lines. Obviously, the relative dimensions of the object surface 32 and the tuhe 3! may be such that the projected area of the object (as projected from center of curvature Cl is less than that of the maximum tube diameter.

I claim as my invention:

1. In a projection system, an object to be projected upon a projection screen, a concave spherical mirror positioned to project said object upon said screen, and a correcting plate positioned at least approximately at the center of curvature of said mirror and shaped or figured to correct for spherical aberration, said having a nonreflecting central area. which is not substantially less than the projected area of said object on the mirror as projected by parallel lines and which is not substantially greater than either the projected area. of said object on the mirror as projected from the said center of curvature or the first mentioned projected area,- whichever is the greater.

2. In a projection system, an object to be projected upon a, projection screen, light obstructing structure associated with said object, a concave spherical mirror positioned to project said object upon said screen, and a correcting plate positioned at least approximately at the center of curvature of said mirror and shaped or figured to correct for spherical aberration, said mirror having a nonrefiecting central area which is at least as great as the projected area of said ohstructing means on the mirror as projectedby parallel lines. v

3. A projection system comprising a concave in the precedspherical mirror having a nonreflecting central stantially equal to twice the diameter of said screen.

4. An image projection device comprising a light source of finite bi-dimensional image area emitting light according to Lamberts law, light obstructing structure associated with said image area, an axially aligned optical system including a concave reflecting surface of revolution positioned to receive the light issuing from the source. and an aspherical zone plate positioned external to the path of the light projected from the light source to the reflector and positioned to receive the reflected light from the reflector, said zone plate being adapted to correct for spherical aberration introduced by the reflecting surface whereby the optical system projects a sharply defined enlarged image substantially free from spherical aberration upon a viewing surface located at a finite distance from the zone plate, said reflecting surfacehaving a non-reflecting central area which is at least as great as the projected area of said obstructing means on the reflecting surface as projected by parallel lines.

- EDWARD G. RAMBERG. 

